Sometimes a nightmare is just a bad dream—isolated and disagreeable imagery with little emotional response from the dreamer. However, a nightmare can lead to feelings of fear, terror, and anxiety; awakening the individual and causing disturbing emotional response, including insomnia, other difficulties in the sleep cycle, or even daytime distress. Fever, ill health, or poor diet can also bring on nightmares.
Normally, people dream more than two hours a night, and a nightmare usually happens in the later hours of REM sleep, or rapid eye movement sleep. The dreamer often awakens from a nightmare with a good recollection of the imagery and content. Some researchers call negative dreams “threat rehearsals,” where we rehearse the possible threats we encounter in real life; other researchers say that people are working through upsetting events of the day. Most garden variety nightmares are stress related. However, if nightmares become frequent to the point of dysfunction, the individual may be suffering Nightmare Disorder (formerly Dream Anxiety Disorder). Nightmares are more prevalent among girls than boys, with occurrences starting before age 10. Children and adolescents tend to suffer more, with less frequency among adults. About half of the adult population experience nightmares on occasion.
The body cycles through different stages of sleep, from light sleep to the deep sleep of REM. REM sleep happens through signals from the pons, which is located at the base of the brain. This is, in fact, where signals for REM sleep originate and where signals to the spinal cord shut off. This is why the body doesn’t move during deep sleep; but if the pons does not shut down these signals, the individual may act out the dream physically—known as REM sleep behavior disorder. If the individual is running in the nightmare, for example, he may well start running literally.